Release sheet for use with multicomponent reactive urethane systems and method of manufacture

ABSTRACT

A flexible composite release sheet providing a replicative surface with a desired surface effect and a method of manufacturing such a release sheet is disclosed. The release sheet is suitable for use in multicomponent reactive urethane casting systems. The invention further provides flexible composite release sheets having a first acrylic functional coating layer containing the desired surface effect and a second silicone release coating layer overlying the acrylic functional coating layer.

BACKGROUND OF THE INVENTION

The present invention relates to a flexible composite release sheetproviding a replicative surface with a desired surface effect. Moreparticularly, the present invention relates to a composite release sheetproviding a replicative surface with a desired surface effect for use inmulticomponent reactive urethane systems. The present invention furtherrelates to a method of manufacturing such a release sheet. The term“surface effect,” as used herein, is intended to encompass threedimensional relief patterns, textures, or embossures, and essentiallyflat finishes, such as highly glossy mirror finish.

A number of processes exist in which a plastic film or sheet is formedon or against a release sheet and then separated from the release sheetafter cooling or curing to set the plastic material. Curing, wherenecessary, may be accomplished by heat, by peroxide catalyst, byultraviolet (UV) radiation or by electron beam radiation. The releasesheet provides a surface from which the set plastic material can bereadily separated and imparts to the surface of the plastic material thequality of finish of the release surface. For example, a desiredtextured surface can be provided on the surface of the plastic materialby forming the plastic material on or against a release sheet having atextured surface that is the mirror image of the desired texturedsurface.

One example of such forming processes is “casting”, wherein a resinousmaterial, such as polyvinyl chloride or polyurethane resin, in aflowable state, is deposited or “cast” onto the release sheet surface,heated, cured and cooled to consolidate the resinous material into acontinuous self-supporting film, and stripped from the release sheet.The release sheet is normally provided with a desired surface effect,such as high gloss, texturing or an embossed configuration, and thesurface effect is replicated on the cast film.

A more specialized casting method involves using multicomponent reactiveurethane systems as the resinous material. In general, polyurethanecasting materials are formed by reacting a diisocyanate and a polyol toform a urethane. Film properties of the polyurethane can be tailored forspecific end-uses by varying the proportions and the chemistry of thediisocyanate and polyol components. Most polyurethanes are used in the“neat” or pre-reacted form. Multicomponent reactive urethanes, however,are not pre-reacted before casting. The diisocyanate and polyolcomponents, as well as other additives such as cross-linkers andaccelerators, are applied to the substrate as individual entities. Thereaction occurs in situ.

Release sheets for use in the above-described casting processes aretypically made by coating, treating, or impregnating a paper sheet orother substrate with a release coating comprised of such materials aspolymethylpentene, polypropylene, polyfluorocarbons, silicone oil,thermoset silicone resins, and other conventional release agents.Surface effects on the release sheet are conventionally provided by anyone of a number of techniques. The release coating can be dried to asmooth surface gloss, or surface effects such as texturing or embossingcan be provided in the coating by mechanical means, applied either tothe surface of the paper before coating or to the paper after thecoating is applied.

U.S. Pat. No. 4,289,821 (Gray et al.) and U.S. Pat. No. 4,322,450 (Grayet al.), the disclosures of which are incorporated herein by reference,disclose techniques for producing surface effects in a release coatingon a release sheet for use in casting processes. One method disclosedcomprises applying a coating of an electron beam radiation curablematerial to one surface of a web substrate, pressing the coated side ofthe substrate against a replicative surface having the desired surfaceeffect to cause the coating to conform to the replicative surface,irradiating the coating with electron beam radiation to cure thecoating, and stripping the substrate from the replicative surface withthe cured coating adhered to the substrate. The replicative surface ispreferably a metal roll with either a pattern engraved in its surface ora highly polished smooth surface. An important advantage of thistechnique is that the pattern or finish of the replicative surface isreproduced in the cured coating with essentially 100% fidelity. Thistechnique enables replication of very fine patterns, such as wood grainand leather grain, on the surface of a plastic cast onto the releasesheet.

U.S. Pat. No. 4,311,766 (Mattor) and U.S. Pat No. 4,327,121 (Gray), thedisclosures of which are incorporated herein by reference, discloseelectron beam curable coating compositions comprising acrylic functionalmaterials and silicone release agents. Such coatings may be used, e.g.,in the processes described in U.S. Pat. Nos. 4,289,821 and 4,322,450, toreproduce a replicative surface in a release sheet with substantially100% fidelity.

Release sheets using acrylic functional release coatings have been usedsuccessfully with casting systems that employ, for instance, polyvinylchloride or pre-reacted polyurethane. These release sheets, however,typically do not provide adequate release properties when used withmulticomponent reactive urethane casting systems.

Some release sheets work well with multicomponent reactive urethanecasting systems, such as release sheets manufactured from extrudedpolypropylene or poly-4-methyl pentene. These release sheets, however,tend to lack the fidelity of replication achieved by the release sheetsdescribed in the above mentioned patents. A need remains for releasesheets providing a replicative surface with a desired surface effect atsubstantially 100% fidelity for use in multicomponent reactive urethanecasting systems.

SUMMARY OF THE INVENTION

The present invention provides flexible composite release sheetsproviding a replicative surface with a desired surface effect. Theinvention further provides flexible composite release sheets for use inmulticomponent reactive urethane casting systems. The invention furtherprovides flexible composite release sheets having a first acrylicfunctional coating layer containing the desired surface effect and asecond aqueous silicone release coating layer overlying the acrylicfunctional coating layer. The invention also provides methods formanufacturing such flexible composite release sheets.

In one aspect, the invention provides a flexible composite release sheetcomprising:

a) a substrate;

b) an acrylic functional coating layer provided on at least one surfaceof the substrate, containing said surface effect; and

c) a silicone release coating layer provided on the acrylic functionalcoating layer, such that the continuity of said release layer ismaintained, acceptable release of the cast film from said siliconerelease coating is achieved and alteration of said underlying desiredsurface effect is minimized.

Preferred embodiments include one or more of the following features. Thesubstrate is paper. The acrylic functional coating is preferably anelectron beam polymerized acrylic functional coating. The acrylicfunctional coating preferably comprises an acrylated oligomer, a monomerselected from a group consisting of monofunctional acrylate,multifunctional acrylate and mixtures thereof, and 2% or less by totalweight of the solid coating of a siloxane release agent. The siloxanerelease agent is preferably an aminofunctional siloxane release agent.The aminofunctional siloxane release agent is preferably less than 1%,more preferably less than 0.25% and most preferably completely removedfrom the acrylic functional coating.

The silicone release coating layer preferably comprises 90 parts or lesspolyvinyl alcohol, 100 parts or less of a complex reactiveorganofunctional siloxane release modifier, 90 parts or less of areactive organofunctional siloxane emulsion coating, 10 to 20 parts of acatalyst selected from a group consisting of platinum complex and tincomplex, and 2 to 12 parts of a nonionic surface wetting agentcontaining polyoxyethylene groups. The nonionic surface wetting agent ispreferably a silicone glycol copolymer wetting agent. The catalyst ispreferably a platinum complex catalyst. The term “parts” as used hereinmeans parts on a dry solids basis.

In another aspect, the invention provides a method of manufacturing aflexible composite release sheet by:

a) applying an acrylic functional coating layer to a substrate;

b) pressing the coated side of the substrate against a replicativesurface to cause the coating to conform with the replicative surface;

c) curing the acrylic functional coating layer;

d) stripping the cured coated substrate from the replicative surface;

e) applying a silicone release coating layer on the acrylic functionalcoating layer; and

f) curing the silicone release coating layer.

Preferred methods include one or more of the following features. Thepressing step and curing step are performed simultaneously. The firstcuring step is preferably achieved by radiation curing, most preferablyby electron beam radiation curing. The applying step is preferablyperformed by airbrush coating.

Other features and advantages of the invention will be apparent from thefollowing detailed description, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic side cross section of a portion of a release sheetof the invention;

FIG. 2 is a graph showing the 60° gloss of a film cast on the surface ofthe release sheet of the invention as a function of the amount ofpolyvinyl alcohol in the film; and

FIG. 3 is a graph showing the release value of a cast polymer from thesurface of release sheets as a function of the number of times therelease sheets are reused.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a release sheet 10, comprises a substrate 11, apolymerized acrylic functional coating layer 12 provided on one surfaceof the substrate and providing a desired surface effect, and a siliconerelease coating layer 13 provided on the polymerized acrylic functionalcoating layer 12. The release sheet 10 provides a replicative surfacewith a desired surface effect at substantially 100% fidelity for use inmulticomponent reactive urethane casting systems.

In brief, the polymerized coating layer 12 preferably comprises anacrylated oligomer, a monofunctional monomer, a multifunctional monomerfor crosslinking, and a siloxane release agent at 2% or less by totalweight of the polymerized coating.

Preferred acrylated oligomers include acrylated urethanes, epoxies,polyesters, acrylics and silicones. The oligomer contributessubstantially to the final properties of the coating. Practitionersskilled in the art are aware of how to select the appropriateoligomer(s) to achieve the desired final properties. Desired finalproperties for the release sheet of the invention typically require anoligomer which provides flexibility and durability. A wide range ofacrylated oligomers are commercially available from UCB ChemicalsCorporation, such as Ebecryl 6700, 4827, 3200, 1701, and 80, andSartomer Company, Inc., such as SB-500.

Typical monofunctional monomers include acrylic acid,N-vinylpyrrolidone, (ethoxyethoxy)ethyl acrylate, or isodecyl acrylate(IDA). Preferably the monofunctional monomer is isodecyl acrylate. Themonofunctional monomer acts as a diluent, i.e., lowers the viscosity ofthe coating, and increases flexibility of the coating. Examples ofmonofunctional monomers include SR-395 and SR440, available fromSartomer Company, Inc., and Ebecryl 111, available from UCB ChemicalsCorporation.

Commonly used multifunctional monomers for crosslinking purposes aretrimethylolpropane triacrylate (TMPTA), propoxylated glyceryltriacrylate (PGTA), tripropylene glycol diacrylate (TPGDA), anddipropylene glycol diacrylate (DPGDA). Preferably the multifunctionalmonomer is selected from a group consisting of TMPTA, TPGDA, andmixtures thereof. The preferred multifunctional monomer acts as acrosslinker and provides the cured layer with solvent resistance.Examples of multifunctional monomers include SR-9020, SR-351, SR-9003and SR-9209, manufactured by Sartomer Company, Inc., and TMPTA-N, OTA480and DPGDA, manufactured by UCB Chemicals Corporation.

The composition may include a reactive or nonreactive silicone,preferably an aminofunctional siloxane, as a release agent. The releaseagent is added to ensure release of the acrylic functional coating fromthe replicative surface which imparts the desired surface effect to thepolymerized coating. Siloxanes are commercially available fromGoldschmidt Chemical Corp., e.g., TEGO Glide ZG-400 and TG RC-704, fromDow Corning Corporation, e.g. 2-8577 Fluid, and from UCB ChemicalsCorporation, e.g., Ebecryl 350.

The composition may also include additives. Typical additives includepigments, fillers, defoamers, adhesion promoters, flatting agents,wetting agents, slip aids and stabilizers. In addition, viscositycontrol additives, such as colloidal silica or volatile solvents, orsurface texture materials, such as starch grains or silica, may beincluded. Moreover, pigments or filler materials such as calciumcarbonate, titanium dioxide, clay, silica, and the like may be includedto reduce costs of the coating or to create an opaque effect. Ifultraviolet radiation is used to cure the acrylic functional coating,the coating must also include a photoinitiator, e.g., Ebecryl BPO andEbecryl 7100, commercially available from UCB Chemicals Corporation.

Preferably, the acrylic functional coating 12 comprises, before curing,10 to 50 parts of the acrylated oligomer, 20 to 60 parts of themonofunctional monomer, e.g., isodecyl acrylate, 20 to 60 parts of themultifunctional monomer, selected from a group consisting of TMPTA,TPGDA, and mixtures thereof, and an aminofunctional siloxane releaseagent at 2% or less by total weight of the acrylic functional coating.More preferably, the aminofunctional siloxane release agent is less than1%, and most preferably at less than 0.25% by total weight of theacrylic functional coating.

When the aminofunctional siloxane release agent is added in amountsgreater than 2% by total weight, the silicone release coating layer 13does not properly adhere to the acrylic functional coating layer 12.Furthermore, the coating spread characteristics of the aqueous siliconecoating are affected, resulting in an unacceptable release andaesthetically undesirable surface effects.

In order for the silicone release coating layer 13 to adhere adequatelyto the acrylic functional coating, not only should (a) the acrylicfunctional coating contain as little of a silicone release agent aspossible while still allowing the acrylic functional coating to releasefrom the surface that imparts to it its surface effect, e.g., 2% or lessby total weight of the acrylic functional coating, but also (b) thesilicone release coating layer 13 should contain a sufficient amount ofa nonionic surface wetting agent containing polyoxyethylene groups toallow the silicone release coating layer to substantially wet thesurface of the acrylic functional coating layer 12, e.g., at least 3% bytotal dry weight of the silicone release coating.

Polyvinyl alcohol is the coating binder for the silicone releasecoating. Polyvinyl alcohol is also used to adjust the gloss of therelease sheet, and, consequently, the gloss of the final product.Polyvinyl alcohol is commercially available from Air Products andChemicals, Inc., e.g., Airvol 107, from E. I. du Pont de Nemours andCompany, e.g., Elvanol 71-30, and from Hoechst Celanese Corporation,e.g., Mowiol 30-92.

Together, the silicone emulsion, the silicone catalyst, and optionally,the silicone release modifier make up a silicone release system thatcontrols the release properties of the release sheet. The reactiveorganofunctional siloxane emulsion coating is the primary releasecomponent and provides stable release properties. The optional complexreactive organofunctional siloxane release modifier may be included toadjust the release of the film from the surface of the release sheet.The silicone catalyst accelerates the curing of the silicone releaselayer. Examples of reactive organofunctional siloxane emulsion coatingsinclude Syloff 22 and Syloff 7910, manufactured by Dow CorningCorporation, and PC-188, manufactured by Rhodia Silicones North America.Examples of complex reactive organofunctional siloxane release modifiersinclude Syloff 7921, manufactured by Dow Corning Corporation, andPC-191, manufactured by Rhodia Silicones North America. Examples ofsuitable catalysts include Catalyst 164, a tin complex catalyst, andSyloff 7922, a platinum complex catalyst, available from Dow CorningCorporation, and PC-95, a platinum complex catalyst, available fromRhodia Silicones North America.

Preferably the catalyst is a platinum complex catalyst. A platinumcomplex catalyst is preferred because lower coating viscosity and lowercoating solids level may be attained without emulsion breakage, whichaffects the continuity of the release surface, resulting in unacceptablerelease and aesthetically undesirable surface effects. Moreover, lowviscosity and solids enables the application of very thin coatings,minimizing the alteration of the topography of the underlying acrylicfunctional coating.

The silicone release coating, used to form silicone release coatinglayer 13, comprises: 90 parts or less polyvinyl alcohol, more preferably70 parts or less; 100 parts or less of a complex reactiveorganofunctional siloxane release modifier, more preferably 50 to 90parts; 90 parts or less of a reactive organofunctional siloxane emulsioncoating, more preferably 50 parts or less; 10 to 20 parts of a catalystselected from a group consisting of platinum complex and tin complex;and 2 to 12 parts (3 to 20% by total dry weight of the silicone releasecoating) of a nonionic surface wetting agent containing polyoxyethylenegroups, more preferably 4 to 8 parts. Preferably the nonionic wettingagent is a silicone glycol copolymer wetting agent.

To achieve proper spread of the silicone coating on the acrylicfunctional coating, at least 3% of a nonionic surface wetting agentcontaining polyoxyethylene groups must be used. When the nonionicwetting agent is added in amounts less than 3% by total dry weight, thesurface tension of the silicone coating remains greater than the surfacetension of the acrylic functional coating, generally creating poorspread. Poor spread results in unacceptable release values and pooraesthetic characteristics. When the nonionic wetting agent is added inamounts greater than 20% by total dry weight, defects, describedvariously as “mottle,” “craters” and “fish eyes,” may appear in thesilicone coating itself. The defects in turn result in unacceptableaesthetic characteristics. Suitable nonionic surface wetting agentsinclude alkylaryl polyether alcohols and, preferably, silicone glycolcopolymer wetting agents. Examples of silicone glycol copolymer wettingagents include Q2-5211 and Q2-5212, manufactured by Dow CorningCorporation. Examples of alkylaryl polyether alcohols include TritonTX-100 and Triton TX-15, manufactured by Union Carbide.

Surface effects can be characterized as falling into two groups: (1) asurface effect consisting of a shallow pattern, such as simulatedleather with a fine grain, or a mirror finish, such as simulated patentleather, which tends to have high gloss; and (2) a surface effectconsisting of a deeper pattern, such as a simulated leather with heavywrinkles, which tends to have low gloss. Gloss of the release sheet isindicated by the 60° gloss of a film cast on its surface, hereinafter“film gloss.” A polyvinyl film is spread on the release sheet surface atconsistent thickness and then sufficiently cured in an oven. The film isthen stripped from the release sheet. The gloss of the surface of thefilm containing the desired surface effect imparted by the release sheetis measured at a 60° angle using a gloss meter, such as the Proglossgloss meter manufactured by Hunter Associates Laboratory, Inc.

For a given type of surface effect, the amount of polyvinyl alcohol inthe silicone release coating has the greatest effect on the finalproduct gloss, as measured by the film gloss. FIG. 2 plots the filmgloss data as a function of parts polyvinyl alcohol in the siliconerelease coating, as provided in Table 1. The amount of both the complexreactive organofunctional siloxane release modifier and the reactiveorganofunctional siloxane emulsion coating have smaller but notinsignificant effects on film gloss.

TABLE 1 Polyvinyl Alcohol (parts) 60° Film Gloss 10 32.8 17 28.3 45 19.890 17

Therefore, for a surface effect with high gloss, e.g., greater than orequal to 20 film gloss, the silicone release coating preferablycomprises less than 45 parts polyvinyl alcohol and 50 to 80 parts of acomplex reactive organofunctional siloxane release modifier. For asurface effect with low gloss, e.g., less than or equal to 4 film gloss,the silicone release coating preferably comprises 25 to 70 partspolyvinyl alcohol, 60 to 90 parts of a complex reactive organofunctionalsiloxane release modifier and 20 parts or less of a reactiveorganofunctional siloxane emulsion coating.

The type of desired surface effect, shallow or deep pattern, alsoaffects the coat weight of the acrylic functional coating because it isthis layer that provides the replicative surface. A deeper pattern willrequire a thicker layer of the acrylic functional coating. Averageroughness height values, as determined using the Sheffield Profilometermanufactured by Giddings & Lewis Sheffield Measurement, provide anindication of the depth of the pattern. Therefore, for a shallowpattern, e.g., 290 microinches (7.4 microns), the coat weight of theacrylic functional coating is preferably 25 to 35 lb/3300 ft² (37 to 52g/m²). For a deep pattern, e.g., 725 microinches (18.4 microns), thecoat weight of the acrylic functional coating is preferably 35 to 55lb/3300 ft² (52 to 81 g/m²).

It is very important to control the coat weight of the silicone releasecoating layer 13 for several reasons. If the coat weight is too low, thecontinuity of the release surface could be interrupted, resulting inunacceptable release. On the other hand, if the coat weight is too high,the surface effect of the underlying acrylic functional coating may bealtered, resulting in significantly less than 100% fidelity. Preferably,the coat weight of the silicone release coating is less than 2.5 lb/3300ft² (3.7 g/m²). Moreover, film gloss is affected by coat weight.Therefore, for a surface effect with low gloss, the coat weight of thesilicone release coating layer 13 is most preferably 0.5 to 1.5 lb/3300ft² (0.7 to 2.2 g/m²). For a suface effect with high gloss, the coatweight of the silicone release coating layer 13 is most preferably 1.0to 2.5 lb/3300 ft² (1.5 to 3.7 g/m²). Table 2 shows that there is nosignificant difference in average roughness height values, before andafter the silicone release coating layer is applied, maintaining thehigh level of fidelity. There is substantially no change in thetopography of the underlying acrylic functional coating layer 12 withthe addition of the silicone release coating layer 13.

TABLE 2 Acrylic Functional Layer With Silicone Layer Pattern(microinches) (microinches) Deep 717.6 ± 46.5 725.4 ± 41.8 Shallow 287.0± 25.5 290.9 ± 30.0 Note: Average roughness height values measured withSheffield Profilometer; Values are an average of 20 readings; andSilicone release coating layer coat weight is 1.1 lb/3300 ft²(1.6 g/m²).

The invention provides a number of advantageous properties. A releasesheet functions as a temporary mold and is typically reused a number oftimes before being discarded. The release values in Table 3 weregenerated by stripping a cast film from the surface of the release sheetusing an Osgood-Sutermeister release tester. The tester provides acomparative measurement of the energy required to strip a 3.8 cm by 7.7cm film sample. Reuse release data are generated by measuring therelease energy after a predetermined number of castings on the samerelease sheet. With each reuse, release values typically increase,eventually reaching unacceptable release values. FIG. 3 graphs the dataprovided in Table 3. To simulate the release of multicomponent reactiveurethane casting systems, a hard releasing aromatic polyurethane filmwas cast on the release sheet. The release values of the release sheetof the invention remain relatively flat while the values for the controlsheet increase with reuse. The control is a conventional release sheetknown to release multicomponent reactive urethane casting systems.Stable reuse release values improve productivity because changes in theprocess in response to changes in release are not required.

TABLE 3 Release at 1^(st) Casting Release at 5^(th) Casting Sample(J/m²) (J/m²) Invention 19.9 18.7 Control 30.3 58.7

Another property of the invention is the absence of a post-cure changein release. Silicone release sheets often require an “aged” release testbecause the release will change as the silicone coating continues tocure completely after manufacture. The release properties of the releasesheet of the invention remain substantially constant after cure. Thelack of this post-cure release change allows for a valid releaseassessment of the release sheet during production.

The substrate 11 may be any type of sheet-like substrate, e.g., paper,metal foil, and plastic film, preferably paper. The substrate should begenerally impervious to penetration of the acrylic functional coating tomaximize the efficiency of the acrylic functional coating. The substrateis preferably paper with a base coat to prevent penetration of theacrylic functional coating. Most preferably, the base coat is a claycoating at a coat weight of approximately 6 lb/3300 ft² (8.9 g/m²).

The most preferred method of applying and curing the acrylic functionalcoating providing the desired surface effect is disclosed in theaforementioned U.S. Pat. Nos. 4,289,821 and 4,322,450. In brief, themethod comprises applying a coating of an electron beam radiationcurable composition (the acrylic functional coating) to one surface of apaper web, pressing the coated side of the paper against a replicativesurface having the desired surface effect to cause the coating toconform to the replicative surface, irradiating the coating withelectron beam radiation to cure the coating, and stripping the paperfrom the replicative surface with the cured coating adhered to thepaper.

The replicative surface is preferably a metal roll with either a patternengraved in its surface or a highly polished smooth surface. Thereplicative surface is preferably provided in the coating by a rotatingendless surface, such as a roll, drum, or other cylindrical surface,which can be revolved past an electron beam curing device, and, ifdesired, a coating station. The coating can be applied directly to thepaper, before the paper engages the roll, or it can be applied directlyto the roll, in which case the paper is pressed against the coated roll.

The acrylic functional coating may be cured by thermal curing, electronbeam radiation, or UV radiation. Electron beam radiation is preferredbecause it can not only penetrate opaque substrates such as paper butalso the thick coatings required for certain desired patterns. Otherforms of radiation curing such as UV radiation can only penetrateoptically clear substrates and not into thick coatings. Electron beamradiation units useful in the present invention are readily availableand typically consist of a transformer capable of stepping up linevoltage and an electron accelerator. In one type of machine theelectrons are generated from a point source, or single, filament andthen scanned electromagnetically to traverse the coated object. Inanother type of machine, the electrons are generated in a curtain froman extended filament, or multifilament, which can irradiate the entirewidth of the surface without the need for scanning. The entire curingstation is enclosed in a lead lined enclosure to prevent stray radiationfrom leaving the curing station area. It is common when curing coatingswith electron beam radiation units to take steps to eliminate oxygenfrom the surface of the coating. In the present apparatus, a nitrogenatmosphere can be applied. Manufacturers of electron beam radiationunits include Energy Sciences, Inc. and RPC Industries.

The silicone release coating may be applied by a variety of coatingtechniques. Examples of coating techniques include, but are not limitedto, bent blade, bevel blade, rod, roll, short dwell, curtain coating,air knife, and air brush. The invention requires a thin layer of thesilicone release coating to maintain substantially 100% fidelity and theaesthetics of the surface effect in the underlying acrylic layer, and tomaintain the continuity of the silicone release surface to ensureacceptable release and uniform aesthetic characteristics. Consequently,the most preferred coating technology is the air brush which provides athin but uniform coating layer.

The silicone release coating may be cured by a variety of curingtechniques. Examples of curing techniques include convection,conduction, infrared, and combinations thereof. Regardless of the curingmethod, it is very important to obtain sufficient silicone cure.Improper silicone cure will affect not only post-cure release, but alsoreuse release values and film gloss. Although cure times andtemperatures will vary depending on the particular product and processequipment, a minimum web temperature of 335° F. (168° C.) should bemaintained for proper cure. FIG. 3 demonstrates the effect of impropercure on reuse release values.

Other embodiments are within the claims. Various modifications of thisinvention will become apparent to those skilled in the art withoutdeparting from the scope or spirit of this invention.

What is claimed is:
 1. A release sheet for use in replicative casting ofcurable systems comprising: a) a substrate constructed to function as atemporary mold during casting of a curable material; b) an acrylicfunctional coating layer provided on at least one surface of thesubstrate, the acrylic functional coating layer including a surfaceeffect to be replicated during casting; and c) a silicone releasecoating layer, in contact with an exposed surface of the acrylicfunctional coating layer, adapted to receive the curable material, toprovide acceptable release of a cast film of the curable material, andto allow the surface effect to be substantially replicated in the castfilm.
 2. The release sheet of claim 1 wherein the acrylic functionalcoating layer comprises an acrylated oligomer and a monomer selectedfrom a group consisting of monofunctional acrylates, multifunctionalacrylates and mixtures thereof.
 3. The release sheet of claim 2 whereinthe acrylic functional coating layer further comprises 2% or less bytotal weight of the coating of a siloxane release agent.
 4. The releasesheet of claim 2 wherein the acrylic functional coating comprises 2% orless by total weight of the coating of an aminofunctional siloxanerelease agent.
 5. The release sheet of claim 2 wherein the acrylicfunctional coating comprises 1% or less by total weight of the coatingof an aminofunctional siloxane release agent.
 6. The release sheet ofclaim 2 wherein the acrylic functional coating comprises 0.25% or lessby total weight of the coating of an aminofunctional siloxane releaseagent.
 7. The release sheet of claim 2 wherein the acrylic functionalcoating is substantially free of an aminofunctional siloxane releaseagent.
 8. The release sheet of claim 2 wherein the silicone releasecoating layer comprises 90 parts or less polyvinyl alcohol, 100 parts orless of a complex reactive organofunctional siloxane release modifier,90 parts or less of a reactive organofunctional siloxane emulsioncoating, 10 to 20 parts of a catalyst selected from a group consistingof platinum complex and tin complex, and 2 to 12 parts of a nonionicsurface wetting agent containing polyoxyethylene groups.
 9. The releasesheet of claim 8 wherein the catalyst is a platinum complex catalyst.10. The release sheet of claim 8 wherein the nonionic surface wettingagent containing polyoxyethylene groups is a silicone glycol copolymerwetting agent.
 11. The release sheet of claim 1 wherein the siliconerelease coating layer comprises 70 parts or less polyvinyl alcohol, 50to 90 parts of a complex reactive organofunctional siloxane releasemodifier, 50 parts or less of a reactive organofunctional siloxaneemulsion coating, and 4 to 8 parts of a silicone glycol copolymerwetting agent.
 12. A release sheet providing a desired surface effectfor use in replicative casting of curable systems comprising: a) asubstrate; b) an acrylic functional coating layer provided on at leastone surface of the substrate, containing said surface effect, comprising10 to 50 parts of an acrylated oligomer, 20 to 60 part of amonofunctional monomer, 20 to 60 parts of a multifunctional monomer,selected from a group consisting of TMPTA, TPGDA, and mixtures thereof,and an aminofunctional siloxane release agent at 2% or less by totalweight of the coating; and c) a silicone release coating layer, incontact with an exposed surface of the acrylic functional coating layer,such that the continuity of said release layer is maintained, acceptablerelease of a cast film from said silicone release coating is achievedand alteration of said underlying desired surface effect is minimized.13. The release sheet of claim 12 wherein the acrylic functional coatingcomprises 1% or less by total weight of the coating of anaminofunctional siloxane release agent.
 14. The release sheet of claim12 wherein the acrylic functional coating comprises 0.25% or less bytotal weight of the coating of an aminofunctional siloxane releaseagent.
 15. The release sheet of claim 12 wherein the silicone releasecoating layer comprises 90 parts or less polyvinyl alcohol, 100 parts orless of a complex reactive organofunctional siloxane release modifier,90 parts or less of a reactive organofunctional siloxane emulsioncoating, 10 to 20 parts of a catalyst selected from a group consistingof platinum complex and tin complex, and 2 to 12 parts of a nonionicsurface wetting agent containing polyoxyethylene groups.
 16. The releasesheet of claim 15 wherein the catalyst is a platinum complex catalyst.17. The release sheet of claim 15 wherein the nonionic surface wettingagent containing polyoxyethylene groups is a silicone glycol copolymerwetting agent.
 18. The release sheet of claim 12 wherein the siliconerelease coating layer comprises 70 parts or less polyvinyl alcohol, 50to 90 parts of a complex reactive organofunctional siloxane releasemodifier, 50 parts or less of a reactive organofunctional siloxaneemulsion coating, and 4 to 8 parts of a silicone glycol copolymerwetting agent.
 19. The release sheet of claim 1 or 12 wherein the driedcoat weight of the silicone release coating layer is less than 3.7 g/m².20. The release sheet of claim 12 having a surface effect with a highgloss wherein the silicone release coating layer comprises less than 45parts polyvinyl alcohol and 50 to 80 parts of a complex reactiveorganofunctional siloxane release modifier.
 21. The release sheet ofclaim 20 wherein the dried coat weight of the silicone release coatinglayer is from 1.5 to 3.7 g/m².
 22. The release sheet of claim 12 havinga surface effect with low gloss wherein the silicone release coatinglayer comprises 25 to 70 parts polyvinyl alcohol, 60 to 90 parts of acomplex reactive organofunctional siloxane release modifier and 20 partsor less of a reactive organofunctional siloxane emulsion coating. 23.The release sheet of claim 22 wherein the dried coat weight of thesilicone release coating layer is from 0.7 to 2.2 g/m².
 24. A releasesheet for use in replicative casting of curable systems comprising: a) apaper substrate constructed to function as a temporary mold duringcasting of a curable material; b) an acrylic functional coating layerprovided on at least one surface of the substrate, the acrylicfunctional coating layer including a surface effect to be replicatedduring casting; and c) a silicone release coating layer, in contact withan exposed surface of the acrylic functional coating layer, adapted toreceive the curable material, to provide acceptable release of a castfilm of the curable material, and to allow the surface effect to besubstantially replicated in the cast film.
 25. The release sheet ofclaim 1 or 24 wherein surface continuity of the silicone release layeris maintained.